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Heat tolerance

The most heat-tolerant species are often the most at risk from warming. Tolerance and the ability to raise tolerance run in opposite directions.

Heat kills by unfolding proteins, and organisms defend against it with chaperones that refold them. That defence is expensive enough to be rationed — and the species with the highest heat tolerance are usually the ones with the least room left to raise it.

What heat actually does at the cellular level is straightforward: it shakes proteins out of their folds. A protein’s shape is held by a large number of weak interactions, and past a certain temperature the thermal energy exceeds them, the protein unfolds, and unfolded proteins stick to each other. The defence is a family of chaperones — the heat-shock proteins — that bind partly unfolded proteins and either refold them or route them for destruction. This system is ancient and present in essentially everything alive, and it is one of the more satisfying pieces of cell biology, because the mechanism of the damage and the mechanism of the repair are both legible. What is more interesting is why organisms do not simply run the defence continuously. The answer is cost. Producing chaperones in quantity is expensive, and it commandeers the protein-synthesis machinery that would otherwise be making everything else, so organisms keep the response below the level that would maximise their short-term survival. One consequence is worth stating plainly: prolonged mild heat can do more cumulative damage than a short severe spike, because the spike triggers a full response and the mild exposure does not. And then there is the result that reorganises how thermal risk is discussed. When both the upper thermal limit and the capacity to raise that limit were measured across twenty species of porcelain crab, the two turned out to be inversely related. The most heat-tolerant species had the least ability to become more tolerant. Being the toughest in the assay is not protection; it is a sign that the headroom is gone.

Developed coverage · 41% complete · reviewed 2026-09-03

What this page covers

Thermal limits have been measured across most animal groups, and the heat-shock response is present in essentially all cellular life. The best comparative data come from marine invertebrates and insects, which can be measured in numbers.

Often confused with: Preferring warm conditions, which is about where an animal chooses to be rather than what it can survive; The capacity to acclimatise to heat, which is a different quantity and runs in the opposite direction; Fever, which is a regulated increase rather than a tolerance of imposed heat

Quick facts

What heat does
Unfolds proteins, which then stick to each other
The defence
Chaperone proteins that refold or dispose of the damaged ones
Why it is rationed
It is expensive, and it displaces ordinary protein synthesis
The counter-intuitive part
The most tolerant species have the least capacity to become more tolerant

What heat does, and what answers it

Proteins come unfolded; chaperones fold them back or throw them away.

The proteins that repair heat damage cost enough that organisms do not keep them running. That rationing is why prolonged mild heat can do more damage than a short severe spike.

Well supported

Good evidence backs this, though some details remain open.

Heat-shock protein induction imposes measurable energetic and fitness costs, and expression is regulated below the level that would maximise acute thermal survival. Chronic sub-lethal thermal stress can therefore produce greater cumulative damage than acute exposure that triggers a full response.

Who this applies to
The heat-shock response and its costs are documented across all cellular life.
Studied in
Animalia, Plantae, Fungi, Bacteria
Why we rate it this way, and what the caveats are
Well supportedModerate confidence

The existence and function of the response are thoroughly established; the cost is measured in several systems. The consequence for chronic versus acute exposure is well argued and less directly demonstrated.

How far it can be extended

The chaperone system is deeply conserved and its costs have been measured independently in animals, plants, fungi and bacteria.

Caveats

  • The cost has been quantified in a modest number of systems, mostly laboratory organisms.
  • Laboratory heat shock is sharper than most natural warming, so the induction thresholds measured may not transfer directly to the field.

Still unanswered

  • How much of the fitness cost is the protein synthesis itself and how much is the disruption caused by suspending normal translation.

Last reviewed 2026-09-03

The evidence (1 study)

The rationing has a consequence that runs against intuition. A short severe heat spike triggers the full response, and the organism repairs itself. A long mild exposure — hot enough to be doing damage, not hot enough to trigger much of a response — accumulates unrepaired damage instead. This is one reason why a marine animal in a heatwave can be in more trouble than the peak temperature alone would suggest, and why laboratory experiments that apply a sharp shock may be measuring something gentler than what a warming ocean does.

Organisms hold the heat-shock response below the level that would maximise their short-term survival, because the response competes for the same protein-synthesis machinery as everything else they need to do.

Based on The proteins that repair heat damage cost enough that organisms do not keep them running. That rationing is why prolonged mild heat can do more damage than a short severe spike.

Being the toughest is a risk factor

Twenty crab species, two measurements each, and an inverse relationship nobody expected.

The most heat-tolerant species are often the most at risk from warming, not the least. Tolerance and the ability to raise tolerance further are inversely related, so the toughest animals have the least headroom left.

Well supported

Good evidence backs this, though some details remain open.

Across congeneric species spanning thermal habitats, upper thermal limits and acclimation capacity are negatively correlated: species with the highest critical thermal maxima show the smallest capacity to shift those maxima with acclimation, leaving them with reduced thermal safety margins.

Who this applies to
Demonstrated across twenty porcelain crab species, and consistent with modelling of thermal safety margins in insects across latitudes.
Studied in
Petrolisthes, Insecta
Why we rate it this way, and what the caveats are
Well supportedModerate confidence

Directly measured within one genus, with a clear mechanism-free statistical relationship, and independently consistent with latitudinal modelling. Whether the inverse relationship holds generally across animals is not established.

How far it can be extended

The inverse relationship was measured within one genus and the same pattern of narrow safety margins in the most thermally exposed species emerges independently from latitudinal analyses of insects.

Caveats

  • Demonstrated within one genus of crabs. The generalisation to other animals is plausible and not established.
  • Behavioural avoidance is not captured: an animal that can move into shade has options a laboratory assay does not measure.

Still unanswered

  • Whether the inverse relationship reflects a physiological trade-off or simply that species already at high temperatures have exhausted a shared ceiling.

Last reviewed 2026-09-03

The evidence (3 studies)

The short answer

Are the most heat-tolerant species the safest from warming?

Generally the opposite. What predicts risk is the gap between current conditions and the limit, plus the ability to move the limit — and the species with the highest limits typically have the smallest gap and the least ability.

Two independent lines of evidence arrive at this. Measuring both quantities across twenty porcelain crab species found them inversely related: high tolerance came with low acclimation capacity. Separately, modelling insect thermal safety margins across latitudes found tropical species living closest to their optima, so that the regions warming least are the ones where warming does most harm. The two studies share no species, no method and no data, and they point the same way: margin matters, magnitude does not.

Check it for yourself

Two ways of being at risk from warming, and which one toughness helps with
What is being askedMeasured byDoes high tolerance help?
Can it survive today’s peak?Upper thermal limitYes — this is what tolerance is
How much margin is left?Limit minus current maximumUsually not — tolerant species sit close to their limits
Can the limit be raised?Acclimation capacityNo — it runs the other way
Can it evolve a higher limit?Genetic variation in the populationUnrelated to current tolerance

Diagram

Tolerance and the room to raise it, plotted against each other

Schematic: the direction of the relationship, not plotted values.

The most heat-tolerant species have the least room leftRoom toraise limitCurrent heat tolerance →plenty of headroomtoughest, and most exposedSchematic: the direction of the relationship, not plotted values.Measured across twenty porcelain crab species.
The same explanation in words

A downward-sloping line with points along it. The horizontal axis is a species’ current heat tolerance and the vertical axis is its remaining capacity to raise that tolerance by acclimation. Species at the left have modest tolerance and plenty of headroom. Species at the right have the highest tolerance and almost none left — the toughest in the assay are also the most exposed to further warming. The figure shows the direction of the relationship measured across twenty porcelain crab species and carries no plotted values.

How we know

Measuring both, and finding they run opposite ways

Are the most heat-tolerant species the safest from warming?

Twenty porcelain crab species spanning habitats from high intertidal to subtidal were measured for two separate quantities: the upper thermal limit, and the capacity to raise that limit after a period of acclimation to warmer conditions. The relationship between the two was then tested.

What happened

The two were inversely related. Species with the highest thermal limits had the least capacity to raise them further, leaving them with the smallest remaining safety margins.

What it shows

That tolerance and the ability to shift tolerance are different quantities, and that measuring one tells you the wrong thing about the other. It converts the adaptation-versus-acclimatisation distinction from a matter of vocabulary into a measured prediction about which species are exposed.

What it does not show

It does not establish the relationship beyond this genus. It also cannot capture behavioural avoidance: an animal that can move into shade or deeper water has options a laboratory thermal assay does not measure, and in the field that may matter more than either quantity.

The controls — what makes this evidence rather than a story
  • Congeneric species, so that the comparison is not confounded by deep phylogenetic differences.
  • Both quantities measured in the same animals under the same protocol, rather than assembled from different studies.
  • A wide habitat range, so the relationship can be seen across the span rather than at one end.

From Acclimation capacity underlies susceptibility to climate change

Most animals never reach their thermal limit

They move. Which is the general pattern of this whole subject.

A laboratory thermal limit is measured on an animal that cannot leave. In the field, nearly every animal facing heat does the same thing first: it goes somewhere else — into shade, into a burrow, into deeper water, or into the night. That is why desert animals turn out to be so much less heat-tolerant than their reputation suggests, and it is also why behavioural options are the first thing to consider when a measured tolerance seems too low for the place an animal lives.

Where the pattern continues

The research behind this page

4 studies, newest first. Each one has a page explaining what it found and what it could not show.

2010Journal of Experimental Biology

The physiology of climate change: how potentials for acclimatization and genetic adaptation will determine “winners” and “losers”

Acclimatisation capacity and adaptive capacity are different quantities, they are not correlated across species, and a species can have a wide tolerance range while having almost no ability to shift it.

2008Proceedings of the National Academy of Sciences

Impacts of climate warming on terrestrial ectotherms across latitude

Tropical species live closest to their thermal optima and have the narrowest safety margins, so modest warming reduces their performance while temperate species, living well below their optima, may initially benefit.

2003Science

Acclimation capacity underlies susceptibility to climate change

The species with the highest thermal limits had the least capacity to raise them further.

1999Annual Review of Physiology

Heat-shock proteins, molecular chaperones, and the stress response: evolutionary and ecological physiology

Heat-shock proteins are molecular chaperones that bind partly unfolded proteins and either refold them or route them for disposal.

This page is a stop on a longer route

A guided journey reads several subjects in a deliberate order, with an argument for why one follows another. You can join in the middle.

Where to go from here

Each of these follows from something on this page — a relationship in the evidence, a claim people ask about, or the next mechanism along.

How complete this page is, and what it is still missing

NatureHQ publishes its own gaps. This page is at 41% completeness against what we would call a finished subject, and was last reviewed on 2026-09-03. It carries 2 claims and answers 9 mapped search questions.

  • no research from the last few years is attached — check for newer work
  • more experiments could be explained in plain English
  • no popular claim about this subject has been checked yet
  • The inverse tolerance–acclimation relationship is demonstrated within one genus; its generality is stated as unresolved.
  • Thermal tolerance in endotherms differs substantially from the ectotherm case that dominates this page.
  • Heat tolerance in plants, where the same chaperones operate under different constraints, is not covered.